Process for production of an electrochemical sub-assembly comprising an electrode and an electrolyte, and the sub-assembly obtained in this way

ABSTRACT

The invention concerns a process for manufacturing a double-layer electrolyte/electrode electrochemical sub-assembly comprising a positive composite electrode layer and a superimposed electrolyte layer, said layers each containing an ionically-conducting macromolecular material and having at their interface a physical and chemical continuum between the macromolecular material of the electrolyte and that of the electrode, and the sub-assembly so obtained. The process consists in depositing one of the layers on a support from a fluid phase containing the constituents of this layer, partially cross-linking the layer, and then spreading the other layer on this partially cross-linked layer from a fluid phase containing the constituents of the other layer and capable of dissolving the macromolecular material of the first layer deposited on the support. Application of the double-layer sub-assembly to the fabrication of electrochemical cells in the form of thin layers.

The present invention relates to a process for the production of anelectrochemical sub-assembly, comprising a positive electrode and anelectrolyte, and also the sub-assembly obtained in this way. Thissub-assembly finds particular application for the production of completeelectrochemical generators, of the type as described in the EuropeanPatent No. 013199, these generators essentially comprising a solidpolymer electrolyte composed of a salt in solution in a macromolecularmaterial, such as, for example, an ethylene polyoxide and/or one of itscopolymers, and a composite positive electrode comprised of the productof agglomeration in a composite mass of a material with ion conduction,composed of a salt in solution in a macromolecular compound, of anelectrochemically active material such as, for example, a vanadium oxideor titanium disulfide, and of an electron conductor material, such as,for example, carbon black.

These generators can be assembled in any manner, but they can also beassembled as described in the patent application filed on the same dayby the same applicants and entitled: Multi-Layer Assembly for Carryingout Assembly of a Thin-Layer Generator, the Process of Manufacture ofthis Assembly and Production of the Complete Generator, and theGenerator Obtained in this Way.

In order for such complete generators to demonstrate good performance,both electrochemically and mechanically, it is important that in thetwo-layer sub-assemblies comprised of the electrolyte layer associatedwith the positive electrode, the said layers possess good adhesion toone another and do not possess any preferential orientation of thechains of the macromolecular material at the electrolyte/electrodeinterface. In fact, such a preferential orientation of these chains canresult in problems of de-lamination of the layers, in other wordsseparation and loss of adhesion between the electrolyte and theelectrode, which can result in phenomena of excess voltage at theelectrodes during operation of the generator, and limits thepossibilities of cycling of secondary generators.

On the other hand, during production of these sub-assemblies, it isnecessary to avoid that the salt which is in solution either in theelectrolyte, or in the electrode, and which can be identical in each ofthese layers or different, has an overly high concentration in one ofthese layers. In fact, such a variation in concentration can result inpoor interpenetration of the polymer chains as well as precipitation ofcomplexes rich in salt, this precipitation taking place in the unit inwhich the salt concentration is too high. These complexes, which arecrystalline and poor ion conductors, result in a local loss ofelasticity of the area around them and of its adhesiveness.

Finally, it is necessary that the sub-assembly obtained has good heatresistance, and, for example, that an elevation of temperature duringthe course of a process of assembly or during a final production phaseof this two-layer sub-assembly does not cause the latter to deteriorate,which can happen when using a solvent whose solvent power increases withtemperature.

The invention has the purpose of establishing a process which allowsproduction of an electrochemical sub-assembly of the type describedabove, which meets the conditions stated above.

For this, the invention provides a process for production of a two-layerelectrochemical electrolyte/electrode sub-assembly comprising, in theform of thin films, a layer of a composite positive electrode composedof the agglomeration product of a composite mass of material with ionconduction, composed of a salt in solution in a macromolecular compound,of an electrochemically active material and an electron conductormaterial, and a layer of a solid electrolyte composed of a salt insolution in a macromolecular material with ion conduction, this processconsisting of:

depositing one of the said layers onto a carrier, based on a liquidphase comprising the elements constituting this layer,

implementing partial cross-linking of the layer deposited in this way,and, on this partially cross-linked layer,

depositing the other layer based on a liquid phase comprising theelements constituting the said other layer, this latter liquid phasehaving the tendency to dissolve or swell the macromolecular materialpresent in the layer first deposited on the carrier.

According to a first embodiment of the process, a layer of the liquidphase comprising the positive electrode is first of all deposited on thecarrier, then after partial cross-linking of the said layer, a layer ofthe liquid phase comprising the elements of the solid electrolyte isdeposited on the former, this latter liquid phase having the tendency todissolve the macromolecular material of the composite positive electrodein the absence of cross-linking.

According to another embodiment of the process, a layer of the liquidphase comprising the solid electrolyte is first of all deposited on thecarrier, then after partial cross-linking of the said layer, a layer ofthe liquid phase comprising the elements of the composite positiveelectrode is deposited on the former, this latter liquid phase havingthe tendency to dissolve the macromolecular material of the solidelectrolyte in the absence of cross-linking.

If the macromolecular material with ion conduction which is part of thecomposition of the solid electrolyte and that of the composite positiveelectrode, is sufficiently fluid at working temperatures, for example amacromolecular material with a low molecular weight, to form a liquidmixture with the other elements comprising the solid electrolyte or thecomposite positive electrode, which can be used in the techniques ofcoating the substrate, such a mixture can be used for deposition of thelayer in consideration.

Advantageously, the liquid phase based on which the solid electrolytelayer or the layer of the composite positive electrode is formedconsists of a solution, or, if not all the products are soluble, of adispersion of the elements comprising either the solid electrolyte orthe composite positive electrode, in a solvent of the macromolecularmaterial with ion conduction present in the layer to be formed, and ofthe salt in solution in the said macromolecular material, the formationof the said layer taking place by application of the correspondingsolution or suspension onto the carrier or the other layer, as the casemay be, followed by evaporation of the solvent.

Preferably, the same solvent is used to form the liquid phases whichserve to form the solid electrolyte layer and the composite positiveelectrode layer, respectively.

As the solvent, one can advantageously use an organic compound which canvaporize, capable of dissolving both the macromolecular material ormaterials with ion conduction and the salt or salts in solution in thesaid macromolecular material or materials. Preferably, the solvent usedhas a boiling point below 140 degrees C.

According to a particular embodiment, deposition of the second layer isfollowed by a step of partial cross-linking of this layer.

Partial cross-linking of the layer deposited on the carrier and possiblyof the other layer can be carried out by any known method, for examplecross-linking by photochemical cationic catalysis, cross-linking byirradiation by a flow beam of electrons or other energy radiation,cross-linking by chemical reaction which creates bridges such asurethane, ester, siloxane, aluminoxane bridges. Preferably,cross-linking by a thermal method is used, in other words by elevationof the temperature in the presence of a cross-linking agent,particularly a chemical generator of free radicals, thermally activated,the said cross-linking agent being present in the liquid phase used toform the layer subjected to cross-linking. Such an elevation intemperature can be used for evaporation of the solvent when the liquidphase used for deposition of the electrolyte layer or the compositepositive electrode layer contains one. The chemical generators of freeradicals which can be used for the said thermal cross-linking can beparticularly peroxides, such as benzoyl peroxide, lauryl peroxide,dicumyl peroxide or also azobisnitrile compounds such asazobisisobutyronitrile.

Partial cross-linking according to the invention is classicalcross-linking which creates bridges between the molecular chains. Thiscross-linking effect depends on the number of sites which can becross-linked and the number of free radicals or the grouping responsiblefor bridges, and therefore on the period of cross-linking, on thetemperature as well as the concentration of the generators of freeradicals or groupings which provide bridges, or also on the intensity ofenergy radiation in the case of cross-linking by irradiation. For eachparticular case, these parameters will be determined in such a manner asto obtain the desired degree of partial cross-linking. In particular, inthe case of the layer first deposited on the carrier, it is important tomake sure that cross-linking of the said layer is not too great, beforethe second layer is applied. In fact, overly strong cross-linking willcreate poor interpenetration of the polymer of the second layer, becausethe polymer of the first layer will be in the form of a compact network.Such dense cross-linking will therefore result in a loss of supplenessand a loss of adhesion of the two layers relative to one another, whichcould be detrimental for any use of the two-layer sub-assembly in asubsequent transfer technique.

On the other hand, if a first layer which has not been cross-linked isused to deposit the second layer, the situation is one of moleculardisorganization due to dissolution of the polymer molecules of the firstlayer by the liquid or fluid components of the second layer. Such anabsence of cross-linking will also result in flow, particularly underheat, with this flow effect increasing with time. A possible phenomenonof separation of the layers must also be feared. Furthermore, absence ofcross-linking will result in a lesser range of plastic deformation,which can be very detrimental for secondary generators, because therewill be no effect of compensation for volume variations during cycling.

The carrier used to deposit the first layer can be composed either of ametallic material produced in the form of thin films, for example aslightly ductile material such as aluminum, but it can also be comprisedof a plastic material, this plastic material demonstrating good adhesionwith regard to the electrolyte, or partial adhesion, or poor adhesion.The choice of the quality of adhesion depends on the sequence ofoperations which are to be carried out, and on the fact whether or notthe two-layer electrode/electrolyte sub-assembly is to be removed fromthe carrier.

The invention also concerns an electrochemical sub-assembly producedaccording to the preceding process. For this, the invention provides anelectrochemical sub-assembly comprising, in the form of thin films, alayer of a composite positive electrode composed of the agglomerationproduct in a composite mass of material with ion conduction, composed ofa salt in solution in a macromolecular material, an electrochemicallyactive material and an electron conductor material, and of a solidelectrolyte layer comprised of a salt in solution in a macromolecularmaterial, this sub-assembly being characterized by the fact that itpresents a continuum between the macromolecular material of the positiveelectrode and that of the electrolyte.

This continuum is characterized by the fact that the macromolecularmaterial with ion conduction of the electrolyte, and that of theelectrode, form interpenetrating polymer structures in it.

During implementation of the process according to the invention, thefact of only cross-linking the first layer partially and of using aliquid phase for deposition of the second layer, which is also a solventfor the first layer, makes it possible to obtain at least partialswelling of the macromolecular material of the first layer, withoutdisorganization of the latter. Therefore better penetration of thechains of the second layer and anchoring of these chains in the polymerchains of the first layer is achieved, since these chains of the secondlayer which are contained in the liquid phase containing them and inwhich the macromolecular material of the first layer is also soluble arepractically in intimate contact with the macromolecular chains of thematerial of the first layer. At the end of the operation of forming thesecond layer on the first, and particularly during evaporation of thesolvent, if one is used, a continuum consisting of an interpenetratingpolymer structure is obtained at the interface of the said layers.

As indicated above, once the second layer has been deposited on thefirst, one can proceed with a second partial or complete cross-linking.At the interface of the two layers, this cross-linking therefore takesplace at the chains which are already interpenetrated and consists ofco-cross-linking, which reinforces the physical continuum uniting thetwo layers, the said continuum making separation of the two layersalmost impossible.

This physical continuum to which the chemical continuity connected withthe chemical nature of the macromolecular chains of each layer is addedmakes it possible to obtain a random orientation and a good bond betweenthe macromolecular chains, something which cannot be obtained by using,for example, a pressing method or heat co-lamination method forproduction of the two-layer sub-assembly.

The process of spreading according to the invention makes it possible toobtain two-layer sub-assemblies which, when used for the assembly ofsecondary generators, make possible a significant increase in the numberof cycles, a reduction in the thickness of the electrolyte, particularlythe critical thickness at which dendritic phenomena appear during chargeand discharge cycles. These two-layer sub-assemblies obtained accordingto the invention also make it possible to increase the useful capacitiesrelative to the installed capacities, as well as an increase inavailable power. Furthermore, these sub-assemblies demonstratemechanical properties which make it possible to plan transfer of themonto metallic films, such as lithium, under strong pressure, which makesit possible to obtain good adhesion, without any short-circuit phenomenaappearing, which property is observed even for very low thicknesses ofthe electrolyte.

The invention will be better understood when reading the followingexamples, which are given as an illustration and are not limiting in anyway.

First series of examples:

A lithium electrochemical generator has been produced according to fourdifferent methods, specifically a method according to the invention andother methods according to prior art, the said generator having thefollowing characteristics:

(a) Electrolyte: The electrolyte is a solid polymer electrolyte with athickness of 20 microns, composed of a solution of lithium perchloratein a copolymer of ethylene oxide and allyl glycidyl ether at 80% byweight of ethylene oxide, the perchlorate being present in a ratio of 7%by weight in the copolymer. The molecular weight of the copolymer is onthe order of 100,000 (viscosimetric molecular weight).

(b) Positive electrode: The positive electrode is a composite electrodebase on titanium disulfide, carbon black and a polymer electrolyte withthe same composition as above. The thickness of this electrode is suchthat it allows deposition equivalent to 2 Coulomb per square cm, inother words 11.6 g per square m of titanium disulfide.

(c) Negative electrode: The negative electrode is comprised of a lithiumsheet with a thickness of 75 microns.

First Method: Preparation of the elements separately.

(a) Preparation of the electrolyte: Using a mold, a solution, inacetonitrile, of a mixture of copolymer and lithium perchlorate in aratio of 7 g perchlorate for 100 g copolymer is spread onto a film ofpolyethylene or polypropylene. The unit is then dried in pulsating airat 80 degrees C. for 5 to 10 minutes, and a transparent film of solidelectrolyte is obtained, with a thickness of 20 microns, on thepolypropylene film.

(b) Production of the positive electrode: The positive electrode isproduced on a film of aluminum with a thickness of 20 microns, doctorblade coating of a suspension of titanium disulfide and carbon black inthe solution of the above-mentioned electrolyte, in such a way as toobtain a composite mass containing approximately 22 g per square m oftitanium disulfide, after drying.

(c) Production of the two-layer sub-assembly: Using a press or a heatinglaminator, the unit of the positive electrode and the electrolyte ispressed at a temperature between 50 and 100 degrees C., under such apressure that the total thickness must be less than the sum of thethickness of the initial components by 10 microns.

The unit called a "half-battery" is obtained; it adheres sufficiently tothe positive collector to allow removal of the polypropylene film havingserved as a carrier during production of the electrolyte. When thispolypropylene film has been removed from the electrolyte, pressing ofthe lithium sheet onto the electrode/electrolyte sub-assembly is carriedout in a second operation, at temperatures and pressures on the sameorder as for transfer of the electrolyte onto the electrode. Anothermethod could consist of first proceeding to transfer the electrolytecarried on the polypropylene film onto a lithium film, then removingthis polypropylene film and finally carrying out transfer of the lithiumelectrolyte unit to the positive, itself carried by a metallic filmwhich comprises a collector.

The generators obtained according to this first method of production arecut off and subjected to cycling tests. It is found that at a cyclingtemperature of 30 degrees C., a cycling program equal to C/20, andcapacity utilization in the vicinity of 30% for the first cycle, it isimpossible to obtain more than three correct cycles, then the phenomenaof the appearance of dendrites and adhesion losses are very rapidlyobserved. Furthermore, during assembly of the complete cells bylamination of the lithium film onto the two-layer electrolyte/electrodesub-assemblies, short-circuit phenomena are observed, which result fromhot flow of the macromolecular material, which can be explained by thefact that the said material has not been cross-linked. Even when takingevery precaution during assembly, a high rate of cells cannot be usedbecause of permanent short-circuits.

Second Method:

(a) Production of the positive electrode: The exact same process whichwas used for production of the electrode described above is used, withthe exception that before the suspension is spread by doctor bladecoating, a free radical generator comprised of benzoyl peroxide in aratio of 0.5% by weight relative to the polymer is added to thesuspension.

Then a composite electrode analogous to the electrode of Example 1 isobtained, except that after drying for 5 to 10 minutes at a temperaturebetween 80 and 100 degrees C., the copolymer is partially cross-linkeddue to the reaction of the allyl groups among one another in thepresence of the free radicals resulting from heating of the benzoylperoxide.

(b) On the other hand, a solution intended to obtain the solidelectrolyte as described in the first process is prepared, with theexception that a free radical generator is added to the solution, which,in this particular case, is the same one as that used for the positiveelectrode. The said generator is used in a ratio of 0.5% by weight ofpolymer. The electrolyte solution is then spread onto the positiveelectrode produced previously, again by doctor blade coating. Thesub-assembly is dried at 80 degrees C., and, as an illustration, it canbe said that there is obtained a "varnish" that cannot be peeled awayfrom the "painting" if the positive electrode. The sub-assembly is thenassociated with a metallic lithium layer in the same manner as thatexplained in Example 1. The generators obtained in this way were cut toidentical formats as those in the first example, and subjected tocycling tests. These cycling tests took place at a temperature ofapproximately 30 degrees C., and it was possible to carry out 320 cycleswith a utilization rate of 50% of the installed capacity in thepositive.

Third Method:

An attempt was made to implement the same process as that described inthe second method, but without using the cross-linking agent. It wasfound that during deposition of the electrolyte layer onto the positiveelectrode, destructurization of the positive electrode took place, andit was impossible to obtain a proper sub-assembly, due to partialdissolution of the electrode in the presence of the solvent of thesecond layer.

Fourth Method:

If one attempts to implement a process analogous to that described inthe third method, but using an electrolyte solvent which is a poorsolvent for the electrode material, for example methanol, numerousproblems immediately occur during drying, because the temperatureincrease results in a dissolution of the positive electrode. To remedythis, it would be sufficient to utilize a free radical generator whichwould allow partial cross-linking of the electrode, according to theinvention.

Second Series of Examples:

Several methods to produce an electrochemical generator were tried,where the electrolyte is identical to that of the first series ofexamples, but with a thickness of 60 microns, and the positive electrodecomprises molybdenum dioxide with a capacity of 5 Coulomb per square cmand is deposited onto a metallic nickel film with a thickness of 5microns. This positive electrode has a capacity of 66.5 g molybdenumoxide per square m.

First Method:

Using a method identical to the first method of the first series ofexamples, it is found that it is very difficult to obtain a properassembly without the occurrence of short circuits, in spite of therelatively great thickness of the electrolyte, which is on the order of60 microns. When, in spite of this, it was possible to obtain agenerator capable of functioning, a utilization rate which decreasedrapidly during the first cycles was observed. During utilization at 60degrees C., with a program of C/12, dendritic phenomena appeared duringrecharging, from the fifth cycle on, in spite of the 60 micron thicknessof the electrolyte.

Second Method:

Using the spreading method in which the cross-linking agent is comprisedof azobisisobutyronitrile (AZBN) used in a ratio of 0.5% by weight ofcopolymer and spreading a thickness of 40 microns of electrolyte on thepositive electrode, and making secondary generators with the same sizeas above, it was found that it was possible to obtain discharges at morethan 70% utilization, for a discharge speed of C/12 at 60 degrees C.,this at a high number of cycles (165). During these cycles, noappearance of dendrites was observed, in contrast to the precedingexample.

If the cross-linking agent had not been used in the positive,disorganization of the latter would have occurred, resulting inseparation or local separation of the positive electrode from itscollector due to certain local dissolution. These problems are resolvedif one proceeds in the presence of free radicals in the positiveelectrode layer. In the absence of free radicals in the solidelectrolyte layer, there are no particular spreading problems, butcertain problems can appear if transfer takes place under heat, forexample to a lithium sheet, these problems showing themselves byshort-circuit effects due to flow effects.

We claim:
 1. A process for production of a two-layer electrochemicalelectrolyte/electrode sub-assembly comprising, in the form of thinfilms, a layer of a composite positive electrode composed of theagglomeration product in a composite mass of material with ionconduction, itself composed of a salt in solution in a macromolecularcompound, of an electrochemically active material and an electronconductor material, and a layer of a solid electrolyte composed of asalt in solution in a macromolecular material with ion conduction, thisprocess being characterized by the fact that it consists of:depositingone of the said layers onto a carrier, based on a liquid phasecomprising the elements constituting this layer, implementing partialcross-linking of the layer deposited in this way, and, on this partiallycross-linked layer, depositing the other layer based on a liquid phasecomprising the elements constituting the said other layer, this latterliquid phase having the tendency to dissolve or swell the macromolecularmaterial present in the layer first deposited on the carrier.
 2. Aprocess according to claim 1, characterized by the fact that a layer ofthe liquid phase comprising the positive electrode is first of alldeposited on the carrier, then after partial cross-linking of the saidelectrode layer, a layer of the liquid phase comprising the elements ofthe solid electrolyte is deposited on the former, this latter liquidphase having the tendency to dissolve the macromolecular material of thecomposite positive electrode in the absence of cross-linking.
 3. Aprocess according to claim 1, characterized by the fact that a layer ofthe liquid phase comprising the solid electrolyte is first of alldeposited on the carrier, then after partial cross-linking of the saidlayer, a layer of the liquid phase comprising the elements of thecomposite positive electrode is deposited on the former, this latterliquid phase having the tendency to dissolve the macromolecular materialof the solid electrolyte in the absence of cross-linking.
 4. A processaccording to one of claims 1 to 3, characterized by the fact that themacromolecular material with a low molecular weight, which enters intothe composition of the solid electrolyte and/or that of the compositepositive electrode is sufficiently fluid at working temperatures, toform a liquid mixture with the other elements comprising the solidelectrolyte or the composite positive electrode, which can be useddirectly, without adding any solvent, in the techniques of coating thesubstrate, the said mixture being used for deposition of the layer inconsideration.
 5. A process according to one of claims 1 to 3,characterized by the fact that the liquid phase based on which the solidelectrolyte layer and/or the layer of the composite positive electrodeis formed consists of a solution, or, if not all the products aresoluble, of a dispersion of the elements comprising either the solidelectrolyte or the composite positive electrode, in a solvent ofmacromolecular material with ion conduction present in the layer to beformed, and of a salt in solution in the said macromolecular material,the solvent present in the liquid phase serving to form the layerdeposited second also being a solvent of the macromolecular materialpresent in the layer first deposited on the carrier.
 6. A processaccording to claims 1 to 3, characterized by the fact that it comprisesa step according to which the layer deposited second is partiallycross-linked.
 7. A process according to claim 6, characterized by thefact that each cross-linking is carried out by one of photochemicalcationic catalysis, irradiation with an electron beam, irradiation withenergy radiation other than an electron beam, and by a Chemical reactionwhich creates bridges.
 8. A process according to claim 6, characterizedby the fact that each cross-linking is carried out by elevation of thetemperature in the presence of a cross-linking agent which is thermallyactivated, said cross-linking agent being present in the liquid phaseused to form the layer subjected to cross-linking.
 9. A processaccording to claim 8, characterized in that the liquid phase used fordeposition of the electrolyte layer of the composite positive electrodelayer comprises a solvent and in that elevation of temperature performedwhen carrying out each cross-linking is used to evaporate said solvent.10. A process according to claim 8, characterized in that thecross-linking agent which is thermally activated is a chemical generatorof free radical.